Evaporative condensing is still by far the most economical means to remove latent heat. Other condensing methods are based on using dry air or a cooling tower. However, this holds true as long as the heat transfer surfaces on both sides of the tubes are kept clean and free of thermal insulating films such as oil, scale, algae growth, etc.
The basic equation for sizing any heat exchanger is: EQU Q=LMTD.times.F.times.U (Equation No. 1)
where,
Q=total heat transferred between the fluids on either side of the pipe walls (BTU/Hour)
LMTD=log mean temperature difference between the fluids (degrees F.) PA1 F=heat transfer surface (in square feet) PA1 U=overall heat transfer coefficient or specific thermal capability of the heat exchanger (BTU/hr.-Ft.sup.2 -.degree.F.) PA1 Hw=the film factor of the water wetting the outside of the tubes. PA1 Lo=the thickness of the oil film. PA1 Lp=the thickness of the tube material. PA1 Ls=the thickness of the scale on the outside of the tubes. PA1 Ko=the conductivity of the oil. PA1 Kp=the conductivity of the tube material. PA1 Ks=the conductivity of the scale deposit.
The heat transfer surface (F) is a function of the coefficient U and shall vary inversely with U.
The LMTD is a function of the cycrometric conditions of the outside air entering evaporative condensor as well as the ratio of the air flow versus the refrigerant to be condensed.
Cycrometric conditions involve humidity and temperature of the air e.g., cycrometric conditions are the outside air.
Therefore, once the designer has set the value of LMTD, the amount of heat transfer surface required will be defined by the value of U. The ability to convey heat between both fluids is equal to the reciprocal of the summation of all thermal resistances encountered: ##EQU1##
A typical evaporative condensor arrangement is shown in FIG. 1. The hot vapor to be condensed reaches a distribution header 31 and is introduced into the pipes which comprise the heat exchanger assembly 10. The condensed liquid inside of the tubes will flow down by gravity into the liquid header 32. Fresh outside air is constantly flowing through the unit. A pump 5 draws water from the basin 4 and takes it to nozzles 3 where it is sprayed over heat exchanger 10. This water picks up heat from the external surface of the pipes and surrenders it to the air by vaporizing a small fraction of its total mass. This process is termed evaporative and there is simultaneous transfer of heat and mass between both fluids, air and water as they come into direct contact with each other.
The thermal resistance R1, R2, R3 as indicated in equation No. 2 have been replaced in equation No. 3 by their corresponding physical properties: ##EQU2## where,
Hr=film factor corresponding to the condensing refrigerant inside the tubes.
The heat exchanging process commences in the inside of the tubes and makes its way to the outside. In any evaporative condensor there are four distinctive stages of the cooling process.
Stage 1 convection of the heat from the vapor inside the tubes to the tube wall,
Stage 2 transmission of the heat through the tube wall,
Stage 3 the water which is wetting the external side of the tube absorbs the heat coming through the wall of the tube,
Stage 4 the water releases the heat to the surrounding air.
Equation No. 3 covers the overall coefficient U for stages 1, 2, and 3.
Stage 4 is the evaporative stage of the heat exchanging process. Here the external surface of the tubes of the heat exchanger 10 are only a part of the total evaporative surface. Evaporative surface is made up of the said tube services plus the curtains of water and droplets which fall all the way down into the basin 4.
The object of the present invention is to obtain the highest or best heat transfer conditions for each and all stages since whichever stage has the lowest value that stage shall define the overall heat transfer capability of the entire evaporative condenser. According to the invention, by raising the efficiency of latent heat removal, the physical size of the overall structure can be reduced.